U. S. Pat. Nos. 3,872,379 and 3,887,865, which are commonly assigned herewith, disclose transducers of so-called "monoturn" type for magnetic detection of object flaws. A disc-shaped conductive member is therein provided with an interior central aperture, for passage of a test object through the transducer, and a slot extends radially from the central aperture to the periphery of the disc-shaped member. A coil encircles the disc-shaped member and alternating or pulsed current supplied thereto produces current flow around the inner surface of the central aperture, thereby providing a cyclic, magnetic field which induces eddy currents in the test object. The term "monoturn" is derived from the single turn surface of the disc-shaped member about the central opening.
Considering the energized coil as a primary winding, a secondary winding is applied to the disc-shaped member to generate output signals indicative of test object eddy current characteristics. The secondary winding output signals serve to identify flaws present in the test object evidenced by cognizable departures of such current characteristics from those which are known to identify a flawless standard specimen. The secondary winding can also serve both functions, in which the primary winding is not necessary.
A characteristic of the monoturn, discussed in the '379 patent as follows, is that, for an encircling primary coil of given diameter and ampere turns, a considerable increase in flux density can be obtained in the aperture as the aperture diameter is decreased. A related advantage is that the inductance of the primary coil is lessened over that obtained in other transducers since such inductance is more a function of the cross-sectional area of the aperture, in which the flux is concentrated, than of the diameter of the winding constituting the primary coil.
The monoturn structures of the referenced patents of applicant are of object-encircling type. Offsetting the above advantages, convenience of usage thereof, as against usage of so-called "tangent coils", is accordingly limited. In this connection, tangent coils, not of monoturn type, have heretofore been used in magnetic object examination and have various usage advantages. One advantage is mechanical in that the tangent coil need only access a peripheral portion of the test object, providing simplicity both in coil and coil support structure. A second advantage is operational and relates to signal-to-noise improvement over encircling coils. Thus, considering the weld zone of a welded steel tube, for example, the major noise source is not the weld area, but the remnant, unwelded area of the tube. Use of a tangent coil in registry with the weld area, as against use of an encircling coil, would accordingly enhance signal-to-noise ratio.
Difficulty arises, however, in conforming a tangent coil to the surface of a portion of a test object. Surface conformity, from applicant's viewpoint, is a prerequisite for proper flaw examination. While considered by applicant, the use of a monoturn coil, as heretofore known, as a tangent coil would be ineffective, since it would be lacking in such surface conformity with a portion of a test object, except in the very narrow slot of the conductive member.